The stoma (singular) is surrounded by two guard cells. The guard cell contains a nucleus and chloroplasts. The guard cells are the only cells of leaf epidermis which have chloroplast. The inner wall of each guard cell is thick and elastic. The outer wall is much thinner. The guard cell is surrounded by epidermal cells. The stoma leads into air spaces that surround thin walled mesophyll cells. Water evaporates from the surfaces of mesophyll cells and the air, in the sacs, becomes saturated with water vapors.